1. Field of the Invention
The invention relates to a process for the chlorination of methylsilanes by reaction of methylsilanes with chlorine in the presence of hydrogen chloride, and to an apparatus suitable for carrying out the process.
2. Background Art
It is known that chloromethylchlorosilanes can be prepared by reaction of methylchlorosilanes with chlorine under the action of high-energy light in a liquid and gaseous phase. On this subject, reference may be made, for example, to DE-A 21 50 718 (Bayer A G, published on Apr. 19, 1973), which discloses a process for the chlorination of methylchlorosilanes by reaction of the starting silanes with chlorine, with the methylsilanes being premixed with chlorine in a first zone and this mixture then being exposed to radiation which triggers the photochemical reaction in a second zone, the photochemical reaction being carried out at below the boiling point of the methylsilane to be chlorinated. DE-A 26 14 197 (Dynamit Nobel A G, published on Oct. 13, 1977) describes the continuous photochlorination of methylsilanes by means of chlorine, with the photochemical reaction being carried out at the boiling point of the reaction mixture.
However, the known processes frequently have serious disadvantages, particularly in respect of economics and their ability to be carried out safely.
The present invention improves upon the chlorination of methylsilanes by chlorination with a substoichiometric amount of chlorine at a temperature below the boiling point of the methylsilane, in the presence of a minor amount of hydrogen chloride, the chlorination induced by electromagnetic radiation.
The present invention provides a process for preparing silanes which contain chloromethyl groups and have the formula
(ClyCH3xe2x88x92y)a(CH3)bSiCl4xe2x88x92axe2x88x92b
where
y is 1, 2 or 3, preferably 1,
a is 1, 2, 3, or 4, preferably 1,
b is 0, 1, 2 or 3, with the proviso that the sum a+b is 1, 2, 3 or 4,
which comprises reacting methylsilanes of the formula
(CH3)a+bSiCl4xe2x88x92(a+b)xe2x80x83xe2x80x83(II),
with chlorine in the presence of at least 0.1% by weight of hydrogen chloride, based on the weight of methylsilane of the formula (II) used, under the action of electromagnetic radiation which triggers the chlorination, with chlorine being used in a substoichiometric amount based on the methylsilane of the formula (II), the reaction being carried out at temperatures below the boiling point of the methylsilane of the formula (II).
Silanes of the formula (II) which are used in the process of the invention are methyltrichlorosilane, dimethyldichlorosilane, trimethylchlorosilane and tetramethylsilane.
Although not shown by the formulae (I) and (II), the process of the invention can also be carried out using silanes of the formula (II) in which all or some of the methyl groups have been replaced by other alkyl groups, e.g. ethyl radicals, propyl radicals, butyl radicals, etc., but this is not preferred.
In the process of the invention, the molar ratio of methylsilane of the formula (II) to chlorine is preferably above 2.0:1.0, more preferably in the range from 6.0:1.0 to 31.0:1.0.
In the process of the invention, hydrogen chloride is preferably used in an amount of at least 0.2% by weight, based on the weight of methylsilane of the formula (II) used. Hydrogen chloride is more preferably used in the form of a methylsilane saturated with hydrogen chloride at atmospheric pressure and a maximum temperature of 40xc2x0 C.
The process is preferably carried out at temperatures which are from 5 to 15xc2x0 C. below the boiling point of the methylsilane of the formula (II), and at the pressure of the surrounding atmosphere, i.e. from about 900 to 1,500 hPa. However, the process of the invention can also be carried out at higher or lower pressures.
In the inventive process, the irradiation is preferably performed employing a wavelength range of from 200 to 800 nm, more preferably from 300 to 400 nm. As a radiation source, it is possible, for example, to employ all sources which have been used hitherto in the photochlorination of methylsilanes. The radiation source can be located within or without the reactor. Examples of lamps which can be used for generating the radiation which triggers the photochemical reaction are UVA fluorescent lamps and intermediate-pressure mercury vapor lamps having predominant emission in the range of from 360 to 370 nm.
The radiation employed according to the invention has a preferred radiation density of 15 Einstein/hm2, as can be used, for example, in plants for the industrial preparation of chloromethylsilanes of the formula (I).
The process of the invention can be carried out continuously or batchwise; a continuous process is particularly preferred and will be described in more detail below.
The process can be carried out in all types of reactors which have been used heretofore for carrying out continuous reactions, for example tube reactors, loop reactors and stirred vessels. The reactors used are preferably tube reactors and loop reactors, with tube reactors being particularly preferred. Depending on the reactor type selected, chlorinated reaction products can, if desired, be recirculated to the photochemical reactor. For example, total recirculation of chlorinated reaction products is possible in a loop reactor, while the chlorinated reaction products are preferably not recirculated in the case of a tube reactor.
In the process of the invention, the reaction mixture preferably contains not more than 35% by weight of chloromethylsilane of the formula (I) and more preferably contains from 5 to 15% by weight of chloromethylsilane of the formula (I).
The starting materials chlorine and methylsilane of the formula (II) are preferably introduced into the reactor individually and exposed to the radiation without premixing. Hydrogen chloride is introduced individually into the reactor or may be premixed with the starting material methylsilane of the formula (II). Thus, hydrogen chloride can be premixed with methylsilane of the formula (II) to be chlorinated, recycled methylsilane containing hydrogen chloride from the work-up of crude product can be used, or a reaction mixture containing hydrogen chloride can be employed.
In the process of the invention, the starting materials and hydrogen chloride are preferably introduced concurrently into the photochemical reactor, more preferably in the absence of chlorinated reaction products or with exclusion of reaction mixture, i.e. by use of pure starting materials only.
The reaction is carried out at temperatures below the boiling point of the methylsilane of the formula (II), preferably in such a way that hydrogen chloride formed, due to its solubility in the reaction mixture, is released virtually completely in gaseous form in the reactor. The reaction temperature is adjusted by supplying an excess of methylsilane at a temperature significantly below its boiling point. Remaining hydrogen chloride is removed together with excess methylsilane of the formula (II) in the distillation of the reaction mixture.
In a preferred embodiment of the process of the invention, chlorine, which may be in gaseous or liquid form, and gaseous hydrogen chloride are introduced at a constant rate into a lower portion of a vertical tube reactor illuminated by means of a UV lamp, and an excess of the methylsilane of the formula (II) is also introduced into a lower portion of the reactor, at such a rate that the temperature of the reaction mixture remains below the boiling point of the methylsilane to be chlorinated. The UV lamp may be located outside the reaction tube or in the reaction tube, and the tube reactor is generally uncooled. The gas/liquid mixture leaving the tube reactor is then preferably separated into liquid and gaseous constituents in a receiving vessel or a cyclone.
The liquid reaction mixture obtained in the process of the invention, consisting for the most part of chlorinated methylchlorosilane of the formula (I) in which y=1 and a=1, unreacted silane of the formula (II), more highly chlorinated products, and hydrogen chloride corresponding to its solubility in the reaction mixture, can be subjected to separation processes known per se, for example by distillation in a column, in which case the desired product is then obtained in very pure form. The unreacted methylsilane of the formula (II) can be recirculated to the tube reactor together with hydrogen chloride which is not separated in the receiving vessel or cyclone.
If a loop reactor is employed in the process of the invention, liquid, degassed reaction mixture from the receiving vessel is preferably cooled and partly recirculated to the loop reactor.
Gaseous hydrogen chloride from the receiving vessel or cyclone is preferably reserved for another use.
The process of the invention is most preferably carried out employing a reactor in which a commercial, tubular, intermediate-pressure mercury vapor lamp having a protective quartz tube and a sheathing tube of borosilicate glass (e.g. Duran(copyright)) is installed concentrically in a metallic outer tube into which starting materials and hydrogen chloride flow from below so that the reactor is operated in a flooded state. When the process of the invention is carried out, hydrogen chloride formed at the glass surface of the UV lamp is advantageously released in gaseous form and consequently reduces the laminar boundary layer of the inflowing starting materials and thereby increases the utilizable radiation density of the UV lamp.
The present invention further provides an apparatus for preparing silanes containing chloromethyl groups under the action of electromagnetic radiation which induces chlorination, wherein a tubular intermediate-pressure mercury vapor lamp having a protective quartz tube and a sheathing tube of borosilicate glass (e.g. Duran(copyright)) is located within a metallic outer tube, preferably located concentrically within the outer tube, into which starting materials and hydrogen chloride flow from below so that the reactor is operated in a flooded state.
Examples of silanes of the formula (I) prepared by the process of the invention are (ClCH2)CH3SiCl2, (ClCH2)(CH3)2SiCl, (ClCH2)SiCl3, (ClCH2)(CH3)3Si, (ClCH2)2CH3SiCl and (Cl2CH)(CH3)2SiCl.
The process of the invention has the advantage that it can be carried out in a simple way with high operational safety margin. The process also has the advantage of saving energy by utilization of the heat of reaction for degassing and in the work-up of the reaction mixture by distillation. The process has the further advantage that no separate reactor cooling is required. In addition to the foregoing advantages, the process is advantageous in that silanes containing chloromethyl groups can be prepared at high conversion, which makes it possible for the chloromethylsilanes to be prepared economically on an industrial scale.
The apparatus of the invention has the advantage that it has a simple construction and can be operated in a simple manner. Furthermore, the apparatus has the advantage that it offers high operational safety, and no particular protective measures against high-energy radiation are necessary.
Unless indicated otherwise, the following examples are carried out at the pressure of the surrounding atmosphere, i.e. at about 1,000 hPa, and at room temperature, i.e. at about 23xc2x0 C., or at a temperature which is established on combining the reactants at room temperature without additional heating or cooling. Furthermore, all parts and percentages are by weight unless indicated otherwise.